The present invention relates to a developing device to be used for electrophotographic image formation apparatuses such as printers and copiers.
In electrophotographic image formation apparatuses such as printers and copiers, there has conventionally been used a developing device which develops with toner an electrostatic latent image formed on a surface of a photoconductor to visualize the image. This developing device generally has a developing roller or developing sleeve which rotates while holding charged toner on the outer peripheral surface in a thin layer state, so that toner is fed onto the photoconductor from the developing roller or the like.
Meanwhile, as printers have been wide-spreading in offices rapidly in recent years, users have been becoming increasingly more conscious of image quality and cost. One of image noise is ‘fogging’ which occurs due to deposition of the toner at portions other than the latent image on the photoconductor. This occurs more often due to a deterioration of toner charge amount resulting from a deterioration of the toner within the developing device during endurance time. The deterioration of toner charge amount due to endurance causes not only image deteriorations but also increases in toner consumption, which substantially causes larger burdens of cost on users.
For electric charging of the toner in the developing device, as is a general method, a blade-like member is brought into press contact against the developing roller, and the toner held on the developing roller in a thin layer state is frictionally rubbed by the blade-like member, thereby electrically charged. However, a nip width formed between the blade-like member and the developing roller, if small, would make it hard to impart a sufficient charge amount to deteriorated toner. Thus, For obtainment of larger nip widths, there have been proposed methods for bringing a film-like member into press contact with the developing roller in Japanese Patent Laid-Open Publications Nos. S63-155065, H05-11583, H10-31358, H10-133474 and H11-272069.
However, in the case where the nip width is broadened by using a film-like member, there is a tendency that although the toner charge amount gradually increases while passing through within the nip, yet the toner becomes less charged in downstream-side part within the nip with respect to the rotational direction of the developing roller, so that the charge amount reaches the ceiling and a desired charge amount cannot be obtained. This tendency is considerable particularly with deteriorated toner, causing a problem that the fogging involved in endurance cannot be improved.